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Re: [Xen-devel] Xen 4.3 development update, 15 Oct





--On 18 December 2012 14:28:57 +0000 George Dunlap <George.Dunlap@xxxxxxxxxxxxx> wrote:

* Make storage migration possible
 owner: ?
 status: ?
 There needs to be a way, either via command-line or via some hooks,
 that someone can build a "storage migration" feature on top of libxl
 or xl.

We have this working with qemu-xen, qcow2 and snapshot rebase. At a libxl
level (but not an xl level), everything seems to be there.

Can you describe in more detail how you implement this? Do you have a
script or something?

We have a pile of C code :-)

A script would do something like:

1. Ask qemu to do a live snapshot using snapshot_blkdev putting the
snapshot in a new file on the new storage device. This ensures that all new
writes go to the new storage device.

2. Rebase the snapshot to a null backing file (I think that's qemu-img
rebase with -b '' though we had to submit a couple of lines of patch to
qemu to make it work) which fills the non-written blocks from the new
snapshot from the old base image and breaks the link to the the old base
image.

In our implementation we have a separate hierarchy database from the parent
links stored within the qcow2 files so we get proper usage counting etc.
but that's an 'implementation detail'.

From memory (and at risk of crossing threads) using this device model and
external snapshots you can use qemu-img resize to resize drives live at
least under KVM. Obviously the information that the drive has been resized
needs to get to the guest somehow.

--
Alex Bligh

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